The present invention relates to the fabrication of semiconductor integrated circuits (IC""s). More particularly, the present invention relates to methods and apparatuses for determining wafer trench depth.
One of the operations in the fabrication of IC""s is the etching of trenches into the surface of silicon wafers. This etch operation is typically performed using well known photolithography and plasma etch technology. Generally, the desired depth of these trenches ranges from about 0.1 microns to 3.0 microns with control of the precise depth of the trench being an important consideration.
During the plasma etch process, the etch rate can vary as a function of etching variables such as the chamber component temperature, the chamber conditioning, and the wafer resist age. Improved trench depth control can result if this xe2x80x9cprocess driftxe2x80x9d can be monitored and compensated.
Currently, there are several well know optical techniques used to measure the depths of trenches etched into silicon, such as spectral reflectometery, monochromatic interference, laser triangulation, confocal imaging, and phase contrast.
While these methods are potentially usable, each of them is limited in some way by the physics and specific limitations of the techniques. For example, the interpretation of spectral reflectometery data requires either a prior knowledge of: film thickness"", materials, and refractive indexes, or, complex and error prone xe2x80x9cfittingxe2x80x9d techniques. The inaccuracies in the optical modeling techniques used are reflected as errors in the calculated etch depth. A change or drift in the refractive index of a film could be interpreted as an error in the trench depth and an erroneous control action taken. A trench depth monitoring capability that is excessively sensitive to extraneous variables may actually result in increased variability, and therefore error rate.
In view of the foregoing, what are needed are improved methods and apparatuses for detecting etch trench depth. Further, the methods should be sensitive only to distance, measure an average depth over a reasonable area, and be compact, robust, and cost effective.
The present invention addresses these needs by providing an optically based trench depth detection method. In one embodiment, a first maxima is detected in the intensity of a multi-wavelength light source, a portion of the light being reflected from the top trench surface of a wafer. A second maxima is then detected in the intensity of the multi-wavelength light source, a portion of which being reflected from the bottom trench surface of a wafer. The method further includes determining a maxima peak difference between the first maxima and the second maxima, wherein the trench depth corresponds to the maxima peak separation.
In another embodiment, a system for optically detecting a trench depth is disclosed. The optical trench depth detection system includes a multi-wavelength light source for providing multi-wavelength light to a wafer. The system further includes a light detector for detecting reflected multi-wavelength light. Preferably, the light detector is configured such that it will detect a first maxima in the light intensity from the multi-wavelength light source, a portion of which being reflected from the top trench surface of the wafer. In addition, the light detector is preferably configured such that it will detect a second maxima in the light intensity from the multi-wavelength light source, a portion of which is reflected from the bottom trench surface of the wafer trench. The system is further configured such that the separation between the first maxima and the second maxima corresponds to the trench depth of the wafer trench.
In yet another embodiment of the present invention a method for making an integrated circuit having an optically detectable trench depth is disclosed. The method comprises introducing a substrate into a processing chamber, and creating a plasma within the chamber. A first maxima is then detected in the light intensity of multi-wavelength light, wherein a part of the light is reflected from the top trench surface of a wafer. A second maxima is then detected in the light intensity of multi-wavelength light, a part of which is reflected from the bottom trench surface of a wafer. The method further includes determining a maxima peak difference between the first maxima and the second maxima, wherein the trench depth corresponds to the maxima peak separation. The substrate is etched until the maxima peak separation corresponding to a predetermined trench depth occurs. Thereafter, the substrate is processed through a series of semiconductor processes to form the integrated circuit.
Advantageously, the use of a direct separation measurement technique by the present invention provides a more accurate and robust measurement than techniques using parameter sensitive models.